1. Field of the Invention
The present invention relates to an imaging apparatus, such as a copying machine and a printer, and an imaging method thereof. More specifically, the invention relates to a tandem color imaging apparatus having a plurality of photosensitive drums, and an imaging method thereof.
2. Description of Related Art
Conventionally, imaging apparatuses include those of a type having a plurality of photosensitive drums. An imaging apparatus of this type transfers toner images from individual photosensitive drums to a transfer belt and superpose the images thereon whereby to acquire a color image. In general, in an imaging apparatus of this type, registered marks are formed on the transfer belt and a mark sensor is used to detect them. The detections are performed to enable the toner images, which are formed on the individual photosensitive drums, to be superposed without interposition mismatches on the transfer belt. Then, the results of the detections are used to correct a variety of static positional mismatches caused by positional variations in image creation systems.
However, in rotational driving of components such as driving gears of the photosensitive drums, it is unavoidable that periodical velocity fluctuating components are added to rotational velocities of outer circumferential surfaces of the photosensitive drums. This is attributed to rotation-axis eccentricity of the drums that occurs depending on the manufacturing precision and the like. The occurrence of periodical velocity fluctuating components remains as a problem in achieving high level alignment. This is because the occurrence of periodical velocity fluctuating components causes dynamic positional mismatches, i.e., periodical positional mismatches even after static positional mismatches have been corrected. To overcome the problem, a method has been developed and attempted that detects the phases of individual photosensitive bodies in accordance with registered marks. However, the velocity fluctuation components to be detected in accordance with registered marks are overlap-added with velocity fluctuation components of a transfer-belt driving roller in addition to the velocity fluctuation components of the photosensitive drums. There arises another problem of how to decompose the overlap-added component to detect velocity fluctuation components of the photosensitive drums thereby to obtain precise phases of the photosensitive drums.
Conventionally, various types of methods for detecting a velocity fluctuation component of a transfer belt, which is one of the two components of velocity fluctuation, have been developed. By way of example, according to the techniques disclosed in Japanese Unexamined Patent Application Publication No. 09-62047, multiple marks are preliminarily formed on a transfer belt, and the marks are sensed by sensors at two locations. Thereby, the belt velocity is obtained from the time of movement of the marks between the sensors. By way of another example, techniques according to U.S. Pat. No. 6,393,244, a mark writing means is provided separately from photosensitive drums. The writing means is used to form a mark excluding the velocity fluctuation component of the photosensitive drum. In addition, techniques using electrostatic marks to prevent overuse of toner are also disclosed.
However, any of above mentioned conventional imaging apparatuses and imaging methods thereof requires two sensors to detect the belt velocity. Further, the apparatus disclosed in the latter publication has the dedicated mark writing means and a dedicated mark detecting means. The dedicated means causes increase of costs of the imaging apparatus, and in addition, requires spacing for mounting the individual members.